There has been known a mobile crane including a travelable crane main body and a counterweight carrier capable of traveling together with the crane main body. The counterweight carrier is mounted with a counterweight to increase stability of the crane main body and improve a hoisting ability of the crane. Japanese Unexamined Patent Publication No. H5-208796 discloses an example of the mobile crane including such a counterweight carrier.
The crane disclosed in Japanese Unexamined Patent Publication No. H5-208796 includes a crane main body including a lower traveling body and an upper swing body and a counterweight carrier coupled to a rear portion of the upper swing body via a coupling member.
The lower traveling body includes a main body traveling motor. The lower traveling body is capable of self-traveling with power generated by the main body traveling motor. The upper swing body is mounted on the lower traveling body to be capable of swing around a vertical axis. In the upper swing body, a work device for hoisting work including a boom, a boom guyline, a mast, a carrier guyline, and a hoisting accessory is provided. The crane main body includes a main body swing motor for driving to swing the upper swing body. When the upper swing body swings during the hoisting work or the like, the upper swing body is caused to swing by the power generated by the main body swing motor.
The counterweight carrier is connected to the mast of the work device via the carrier guyline to share a hoisting load applied to the work device. The counterweight carrier includes a plurality of wheels and a carrier traveling motor. The carrier traveling motor rotates the wheels, whereby the counterweight carrier is capable of self-traveling. The wheels are capable of being steered around a vertical axis. During the traveling of the crane main body by the lower traveling body, the carrier traveling motor rotates the wheels in a state in which the wheels are steered such that the orientation of the wheels coincides with the front-back direction of the lower traveling body, whereby the counterweight carrier travels in a direction same as a traveling direction of the crane main body. During the swing of the upper swing body, the carrier traveling motor rotates the wheels in a state in which the wheels are steered such that the orientation of the wheels is along a swing direction of the upper swing body, whereby the counterweight carrier travels in a direction same as the swing direction of the upper swing body.
In the mobile crane explained above, for operation of the mobile crane (the swing of the upper swing body and the traveling of the crane main body by the lower traveling body), the power of the carrier traveling motor can be used in some case and cannot be used in other cases.
For example, in some case, the counterweight carrier receives a hoisting load from the work device for hoisting work and is afloat above the ground. In this case, even if the carrier traveling motor rotates the wheels, since the wheels are not grounded, the power of the carrier traveling motor cannot be used for the swing of the upper swing body and the traveling of the crane main body. On the other hand, in a state in which the wheels of the counterweight carrier are grounded, the power of the carrier traveling motor can be used for the swing of the upper swing body and the traveling of the crane main body.
In the mobile crane, the counterweight carrier is not always coupled to the upper swing body. That is, in some case, the counterweight carrier is detached from the upper swing body and the crane main body independently operates. In this case, the power of the carrier traveling motor cannot be used for the operation of the crane main body.
As explained above, the power of the carrier traveling motor can be used or cannot be used for the operation of the crane main body according to presence or absence of the coupling of the counterweight carrier to the upper swing body and presence or absence of the grounding of the counterweight carrier in a state in which the counterweight carrier is coupled to the upper swing body. When the power of the carrier traveling motor cannot be used, the crane main body needs to operate with only the power of the driving motor (the main body traveling motor or the main body swing motor) of the crane main body. It is likely that operation speed of the crane main body is deteriorated. On the other hand, when the power of the carrier traveling motor can be used, since the power of the carrier traveling motor is added to the power of the driving motor of the crane main body, it is likely that excessive power is obtained.